Method and device for burning disintegrated plant-based fuel

[0002] The invention relates to a method of burning disintegrated plant-based fuel, in which method fuel is burned in a boiler designed for burning dry fuel, fuel is dried with the heat energy obtained from the combustion gases of the boiler and fed after that into a combustion chamber of the boiler for burning.

[0003] The invention relates further to an arrangement for burning disintegrated plant-based fuel, the arrangement comprising a boiler designed for burning dry fuel, a feeding apparatus for feeding fuel into a combustion chamber of the boiler, a drying apparatus for drying the fuel to be fed to the arrangement and means for feeding the heat energy of combustion gases of the boiler into the drying apparatus.

[0004] In fairly small chip burning plants of a size class typically below 2 MW, the use of boilers planned for burning dry chips is most preferable, as far as purchase price is concerned. Such plants are poorly adapted to variations in the moisture of the fuel, i.e. they are not capable of burning for instance, unseasoned chips, bark or sawdust. When boilers are planned in such a way that also unseasoned fuel can be burned in them, the price of the plant easily rises higher. Primary price increasing factors are additional masonry for the combustion chamber, an enlarged convection part and preheaters of combustion air. A plant planned for burning moist fuel is also poorly suited for dry fuel, because dry fuel raises the temperature of the combustion chamber high and the plant will be subjected to resistance problems, for instance. If necessary, dry fuel must even be wetted. A plant planned for burning moist fuel is also poorly suited for small power level requirements because of difficult power control, particularly when dry fuel is used.

[0005] Finnish publication 780 822 discloses a fuel drying apparatus of a boiler. The drying apparatus comprises a fuel tank, from which the fuel is transported to a combustion chamber of the boiler. Combustion gases from the boiler are led into a closed space below the fuel tank, whereby the combustion gases heat the fuel. By this arrangement, moist fuel can be dried before it is led into the boiler, but when already dry fuel is fed into the fuel tank, the combustion gases dry it further completely unnecessarily, which causes a fire risk in the fuel tank, for instance.

[0006] Finnish publication 60 435 also discloses a fuel drying apparatus. The drying apparatus comprises a vertically longitudinal flat chamber heated by combustion gases, in which chamber there are horizontal screw conveyors on each other and under the screw conveyors chutes, in which the fuel passes from one screw conveyor to another back and forth and downwards. The combustion gases are directed to flow upwards in the chamber, whereby they dry fuel. The arrangement is inconvenient and complicated and the drying procedure cannot be controlled in any way, which means that when, for instance, dry fuel is fed it is dried unnecessarily and there is an obvious fire risk in the drying apparatus.

[0007] German Offenlegungsschrift 39 13 885 discloses a boiler arrangement for burning moist fuel. This arrangement comprises a slanting grate, to the upper end of which the fuel is led. The fuel is dried on the upper part of the grate by conducting combustion gases for the purpose of drying. The fuel dries on the drying part of the grate and it is burnt on the remaining part of the grate. This arrangement has a big and expensive structure and it is not suitable for burning dry fuel.

[0008] The object of this invention is to provide a method and an arrangement not characterized by the above-mentioned drawbacks and making it possible to burn disintegrated plant-based fuel of variable moisture.

[0009] The method according to the invention is characterized in that fuel the moisture of which can vary between dry and wet is used and that the combustion gases are conducted to heat air and that heat and flow amounts of this air are dimensioned in such a way that the air in question is sufficient for drying the fuel in all moisture conditions of the fuel and that the heated air is directed in such a way that the air not needed for drying fuel is conducted into the boiler to serve as combustion air.

[0010] Further, the arrangement according to the invention is characterized in that the arrangement comprises a heat exchanger and an air duct, the heat exchanger being arranged to heat the air flowing in the air duct with the heat energy of the combustion gases of the boiler, that the heat exchanger and the air flow amount are dimensioned in such a way that the air flowing in the air duct is sufficient for drying the fuel in all moisture conditions of the fuel and that the arrangement comprises means for conducting the air flowing in the air duct and not needed for drying the fuel into the boiler to serve as combustion air.

[0011] An essential idea of the invention is that the boiler of the arrangement is designed to burn dry fuel and that when moist or wet fuel is fed to the arrangement, the heat energy obtained from the combustion gases of the boiler is utilized for drying the fuel and the heat energy not needed for drying the fuel is directed to heat the combustion air of the boiler. Further, the idea of an embodiment consists in that the combustion gases are conducted to a heat exchanger heating the air which is led either to dry fuel or to serve as combustion air of the boiler, as needed. The idea of another preferred embodiment is that the fuel is dried by a wire dryer, the air heated with the heat energy of the combustion gases being conducted through the wire. The idea of a third preferred embodiment is that a recovery of the heat of drying air and the air flow to be led through the chips are dimensioned in such a way that the air flowing out through the chips is substantially saturated with water vapor. The idea of a further fourth preferred embodiment is that an air distribution either for a purpose of drying fuel or for serving as combustion air of the boiler is controlled on the basis of the temperature in the boiler.

[0012] An advantage of the invention is that fuel of variable moisture can be fed to the arrangement for burning. Another advantage is that the boiler is designed to use dry fuel, due to which the burning procedure is easy to implement technically and the boiler can be provided with an advantageous structure. It is then possible to use, for instance, several different alternative boiler grate solutions, high temperature can be achieved for the combustion chamber, due to which a great part of the heat energy can be recovered as radiation heat, little masonry is needed in the boiler, the fuel can be burnt accurately causing minor outlets, and further, condensation and corrosion problems can be avoided. In addition, the temperature of the combustion gases can be rather high, due to which the convection part of the boiler can be dimensioned to be small and the aging of the boiler is not significant. Moreover, when dry fuel is used, the heat energy of the combustion gases can be recovered through the combustion air back to the system to be utilized. The efficiency of the arrangement is also rather high irrespective of the moisture values of the fuel. Further, the arrangement can be built in such a way that its total price will be low. Likewise, the rather efficient arrangements can be implemented in such a manner that they can be positioned in movable containers, for instance.

[0013] The invention will be described in more detail in the attached FIGURE showing an arrangement according to the invention schematically.

[0014] The FIGURE shows a boiler 1 comprising a combustion chamber 1aand a convection part 1bin a manner fully known per se. Further, the figure shows a feeding apparatus 2 of fuel 18 schematically. The fuel 18 can be fed into the boiler 1 e.g. by a screw conveyor or by using so-called stoker burning, for instance, or some other solution fully known per se. Since the feeding apparatus 2 of fuel 18 is fully known per se, it is not discussed further in this connection. Combustion gases are led out of the boiler 1 through a combustion gas duct 3. The combustion gases are conducted according to arrows A through a heat exchanger 4. The heat exchanger 4 heats the air flowing in an air duct 5. The air flows in the air duct 5 according to arrows B. The air heated by the heat exchanger 4 is conducted through a drying air duct 6 to a drying apparatus 7 to dry moist fuel 18. The heat exchanger 4 can either be a separate heat exchanger or an integral part of the boiler 1.

[0015] The drying apparatus 7 comprises an air permeable wire 8 forming an endless loop and arranged to go around rolls 9aand 9b.A chamber 10 is arranged in the middle of the loop formed by the wire 8. The heated air is conducted through the drying air duct 6 into the chamber 10. The heated air is blown out of the chamber 10 through the upper, permeable support part of the wire 8, whereby the air also flows through the fuel 18 above the wire 8 according to arrows C and D. The fuel 18 is fed on the wire 8 through a feed channel 11, for instance, The fuel 18 may consist e.g. of dry or unseasoned chips, bark, sawdust, grain screenings, waste from carpentry industry, sod peat or some other suitable plant-based disintegrated fuel. The first end of the chamber 10 is preferably sealed against the first roll 9aand the second end against the wire 8. By flowing through the fuel 18, the heated air extracts moisture from it. The temperature of the drying air and the speed of the wire 8 are preferably dimensioned in such a way that the air flowing out through the fuel 18 is substantially saturated with water vapor. The rolls 9aand 9bgoing around, i.e. the movement of the wire 8, may either be continuous or intermittent, and controlled by a step motor, for instance. However, the most essential thing is that the temperature and the outlet velocity of the drying air as well as the movement and surface area of the wire 8 are dimensioned in such a way that the fuel 18 can be dried sufficiently dry.

[0016] In the case according to the attached FIGURE, moist fuel 18 is fed to the drying apparatus of the arrangement according to the invention, whereby all the air heated by the heat exchanger 4 is directed by means of a damper 12 of the drying air duct 6 and a damper 14 of a combustion air duct 13 to dry the fuel 18. Thus, drying air duct 6 and combustion air duct 13 received the heated air flowing in air duct 5. Combustion air is then led from a secondary air duct 15 through the combustion air duct 13 into the boiler. The attached figure shows the procedure of leading combustion air in a simplified manner, but naturally, it is possible to lead both primary and secondary combustion air into the boiler in a manner fully known per se. When the fuel 18 is not so wet that all the air heated by the heat exchanger 4 has to be led to dry the fuel 18, the multipositional damper 12 of the drying air duct 6 is closed partly thus, drying air duct 6 and combustion air duct 13 received the heated air flowing in air duct 5, and the multipositional damper 14 of the combustion air duct 13 is opened partly, thus, drying air duct 6 and combustion air duct 13 received the heated air flowing in air duct 5, whereby part of the heated air is conducted to serve as combustion air of the boiler. Further, when the fuel 18 fed to the arrangement is so dry that no drying at all is needed, the damper 12 of the drying air duct 6 is closed entirely and the damper 14 of the combustion air duct 13 is opened in such a way that all the air heated by the heat exchanger 4 is conducted to serve as combustion air. For the sake of clarity, the attached figure does not show fans required for moving air in the ducts.

[0017] The dampers 12 and 14 can be controlled by a control device 17. Information on the temperature of the combustion chamber 1adetected by a sensor 16 is given to the control device 17. When rather moist fuel 18 changes into drier fuel, the fuel going into the combustion chamber 1aafter the drying apparatus 7 is dryer than before and the temperature of the combustion chamber 1arises. The sensor 16 detects this temperature rise, due to which the control device 17 controls the dampers 12 and 14 in such a way that a greater part than before of the air heated by the heat exchanger 4 will serve as combustion air. When the temperature rises further, the control device 17 closes the damper 12 of the drying air duct 6 entirely and all the combustion air can be preheated. When the fuel changes into a moister fuel than before, the temperature of the combustion chamber la begins to fall and the control device 17 controls the air heated by the air exchanger 4 by means of the dampers 12 and 14 to dry the fuel 18 more than before. The dampers 12 and 14 can also be controlled on the basis of the outlet temperature or outlet moisture of the drying air, for instance, or on the basis of some other suitable basic data.

[0018] The boiler 1 of the arrangement according to the invention is designed for dry fuel, which in this connection signifies that special solutions necessary for burning moist fuel need not be taken into consideration in the structure of the boiler 1. For instance, prices of solutions to feed and grate problems of the fuel may be low, without the reliability of operation suffering, however. The starting point of the designing may, for instance, be that such fuel is dry fuel which always has a moisture of 35% or less. In the structure of the boiler, attention can then easily be paid to the fact that, for instance, fuel of a moisture of 10% or even quite tinder-dry fuel is fed to the arrangement. Calculations have proved that the moisture of the fuel to be fed to the arrangement may be for instance 60%, and nevertheless, the efficiency of the arrangement can be kept rather good. When drier fuels than this are used, the efficiency is better still, of course. When drier fuel is used, the temperature of the combustion chamber 1arises, which intensifies the heat transfer to the boiler.

[0019] The combustion gases can be removed from the boiler 1 in a rather hot state, (e.g., at about 300 to 400° C. degrees, and more preferably 350 to 400° C. degrees, for instance). Then the convection part 1bof the boiler can be made small, and simultaneously, the problems with the arrangement getting dirty are decreased. The temperature of the combustion gases can be dropped by the heat exchanger 4 to about 100° C. degrees, for instance. Then the air in the air duct 5 can be heated by the heat exchanger 4 rather well.

[0020] The FIGURE and the related description are only intended to illustrate the idea of the invention. As to the details, the invention may vary in the scope of the claims. So, prior art techniques can be used for storing fuel and feeding it to the drying apparatus, for instance.